Motor Caculations for Short Circuit and Ground Fault Protection

[Maurice Ibanez]

I am being challenged by a contractor's Electrical Engineer who designed a motor branch circuit for a roll up door. The EE wants the contractor to install a 30 amp circuit breaker to provide short circuit and ground fault protection for a 115 vac 1/2 hp motor, with 8.2 amps FLA on motor data plate. NEC article 240.248 uses 9.8 amps for the computation. I have computed using an inverse time circuit breaker and come up with 20.5 and 24.5 amps respectively thus a 25 amp circuit breaker must be installed to provide short circuit and ground fault protection. The EE is currently trying to add additional loads from the motor control panel to the formula to get over the 25 amp hump since the have already purchased all the required breakers.

I have not found any documentation in the NEC that includes the control panel loads for computing short circuit and ground faults. Is there any articles that I have missed or any documents to support using or not using those loads.

 

[mcclary's electrical]

I am being challenged by a contractor's Electrical Engineer who designed a motor branch circuit for a roll up door. The EE wants the contractor to install a 30 amp circuit breaker to provide short circuit and ground fault protection for a 115 vac 1/2 hp motor, with 8.2 amps FLA on motor data plate. NEC article 240.248 uses 9.8 amps for the computation. I have computed using an inverse time circuit breaker and come up with 20.5 and 24.5 amps respectively thus a 25 amp circuit breaker must be installed to provide short circuit and ground fault protection. The EE is currently trying to add additional loads from the motor control panel to the formula to get over the 25 amp hump since the have already purchased all the required breakers.

I have not found any documentation in the NEC that includes the control panel loads for computing short circuit and ground faults. Is there any articles that I have missed or any documents to support using or not using those loads.

If you have a motor control panel, he can add add as much load to the feeder as he wants, as long as the feeder is rated for it.

 

[kwired]

I am being challenged by a contractor's Electrical Engineer who designed a motor branch circuit for a roll up door. The EE wants the contractor to install a 30 amp circuit breaker to provide short circuit and ground fault protection for a 115 vac 1/2 hp motor, with 8.2 amps FLA on motor data plate. NEC article 240.248 uses 9.8 amps for the computation. I have computed using an inverse time circuit breaker and come up with 20.5 and 24.5 amps respectively thus a 25 amp circuit breaker must be installed to provide short circuit and ground fault protection. The EE is currently trying to add additional loads from the motor control panel to the formula to get over the 25 amp hump since the have already purchased all the required breakers.

I have not found any documentation in the NEC that includes the control panel loads for computing short circuit and ground faults. Is there any articles that I have missed or any documents to support using or not using those loads.

The code may allow for 25 amp the reality is there are many of them installed with 20 amp breaker and even 15 amp breaker and they operate without any trouble. Why is a $3-$7 breaker such a concern just because it has already been purchased. Many supply houses would return or exchange it with no problem.

Or is it some old panel no longer made?

 

[cpal]

a 115 vac 1/2 hp motor, with 8.2 amps FLA on motor data plate. NEC article 240.248 uses 9.8 amps

an inverse time circuit breaker and come up with 20.5 and 24.5 amps respectively thus a 25 amp circuit breaker must be installed to provide short circuit and ground fault protection. .

430.6 (A) directs you to use the table current (9.8A) for sizing the motor branch circuit , so the max SCGF protection is not larger than 24.5 (assuming your cal's are correct) with out applying exceptions. the circuit size is permitted to be increased if the BC is considered to be "not individual" for one motor and it is permissable to increase both conductor and ocpd sizes.

 

[augie47]

Maurice,
Welcome to the Forum.
I am in agreement with you. Without the exception, 430.52 would limit the SCGF protection to a 25 amp breaker (9.8 x 2.5)
If he wants to include other loads then that breaker would be considered feeder protection and you would need proper SCGF protection for the motor downstream.
One other suggestion I might make would be to check for manufacturer's instructions and see what they have to say. 110.3(B) is often the deciding factor.

 

[Maurice Ibanez]

I got them to correct the feeder circuit, now we're not seeing eye to eye on the branch circuit.

 

[Maurice Ibanez]

The code allows for 25 amps they want to install 30 amps. It's costing them $$$ on liquidation damages every day that's why they want to prove the Government wrong.

 

[Maurice Ibanez]

The control panel is part of the motor controls and motor assembly (it's loads are very small) using only 120v/24vac control transformer. The branch circuit is wired into the motor control center where the control circuit is used to close the contactor supplying power to the motor. On larger motors there are larger loads on the mcc however, not significant enough. The OCD is sized to prevent nuciance tripping I would assume that sizing has taken the control loads into consideration. Just don't know how to back up that theory without any references or explanation to pose the the EE.